fix: Handle null partNumber (#1125)

Author: lauzadis

services/s3/e2eTest/src/PaginatorTest.kt

@@ -73,7 +73,7 @@ class PaginatorTest {
                     key = "list-parts-test"
                     uploadId = id
                 }
-                    .transform { it.parts?.forEach { emit(it.partNumber) } }
+                    .transform { it.parts?.forEach { it.partNumber?.let { emit(it) } } }
                     .toList()
                     .sorted()
             }